Greenhouse gases and carbon dynamics of forestry

Summary

Forests are both sources and sinks for several different greenhouse gases (GHG), including carbon dioxide (CO2) that is exchanged during photosynthesis, respiration and decay, methane (CH4) which is released during anaerobic decay processes and nitrous oxide (N2O) which can be released during the cycling of mineral nitrogen. Growing trees takes up CO2 from the atmosphere, and using timber and wood fuel can reduce GHG emissions from fossil fuel combustion.

Therefore, understanding the processes and rates of exchange (or ‘flux’) of these GHG in forests, and how they are affected by forest management is a substantial part of our research. There are several projects in this general topic:

Measurement of CO2, CH4 and N2O fluxes above and below the forest canopy

Methodologies have been developed to take simultaneous measurements of CO2, CH4 and N2O exchange above and below the forest canopy, using both soil chamber and micrometeorological eddy-covariance methods.

Soil chambers have fixed bases installed long-term, to minimise disturbance, and are temporarily closed at fortnightly or monthly intervals with a lid to allow the sampling of the air in the ‘headspace’ over a short period (hour or less). Air samples are then analysed using gas chromatography to estimate rates of GHG efflux or uptake. Records over several seasons are then analysed to estimate annual budgets and to assess the influence of weather variations.

Eddy-covariance methods use continuous sampling of air above the forest canopy at high rates, with sensitive and rapid gas analysers (using Quantum Cascade Laser Absorption Spectrometry) and rapid wind speed and turbulence information recording to estimate exchanges between forest and the air moving above it. We are using these methods to measure CO2, CH4 and N2O fluxes in our long-term oak woodland site (see Straits Inclosure long-term carbon flux site) and at our upland spruce forest site (see Harwood Forest).

Information from these measurement programmes are used to develop quantitative models of GHG exchange, and to assess the effect of different conditions and management techniques.